1. Field of the Invention
The present invention relates to a spline telescopic shaft, a method for manufacturing the spline telescopic shaft, and a vehicle steering apparatus.
2. Description of Related Art
A telescopic shaft for vehicle steering in which a surface hardened layer is provided on a surface of fitted teeth by shot peening processing, and a large number of minute recesses are formed on the surface hardened layer has been proposed (see, e.g., Japanese Patent Application Laid-Open No. 2005-153677). The recesses function as a grease reservoir.
A female spline manufacturing method including a process for forming a groove in a circumferential direction to be a basis of an oil reservoir on an inner peripheral surface of a hollow cylindrical material and a process for forming a female spline by plasticity processing on the inner peripheral surface of the hollow cylindrical material having the groove formed therein has been proposed (see, e.g., Japanese Patent Application Laid-Open No. 2006-207639). The groove is used as an oil reservoir by leaving a trace of the groove on a surface of a tooth surface of an inner alpine formed by the plasticity processing.
In this type of spline telescopic shaft, tooth surfaces of an inner shaft and an outer shaft may be respectively provided with resin coatings. However, backlash in a rotation direction is produced between the inner shaft and the outer shaft due to the effect of a variation in dimensional precision between both the shafts. In order to reduce the backlash, the inner shaft and the outer shaft are selected depending on the dimensional precision and combined with each other so that a fitting clearance between both the shafts becomes suitable (so-called matching assembly).
Even if no backlash is produced between both the shafts in the early stages of use, however, backlash between both the shafts is rapidly increased when a period of time for the use has elapsed.